1. Field of the Invention
This invention relates to a novel and improved anti-lock control system for a motor vehicle, which is operative to prevent the wheels of the motor vehicle from being locked during braking operation of the motor vehicle. More particularly, the present invention is directed to an anti-lock control system which is designed to prevent occurrence of a non-braking condition when anti-lock control is started immediately after driving wheel slipped.
2. Description of the Prior Art
Generally, with an anti-lock control system for a motor vehicle, anti-lock control is effected by means of microcomputers such that hold valves and decay valves comprising electromagnetic valves are opened and closed on the basis of electrical signals representing wheel speeds sensed by wheel speed sensors, thereby increasing, holding or reducing the brake hydraulic pressure, for the purpose of securing improved steering performance and running stability of the motor vehicle, while at the same time shortening the braking distance.
To have a better understanding of the present invention, reference will first be made to FIG. 1 of the accompanying drawings, which is a block diagram illustrating the construction of a conventional four-sensor, three-channel anti-lock control apparatus, which includes left-hand front, right-hand front, left-hand rear and right-hand rear wheel speed sensors 1 to 4. Outputs of these wheel speed sensors 1 to 4 are respectively passed to computing circuits 5 to 8 from which signals representing respective wheel speeds Vw1 to Vw4 are derived. Of the four wheel speed signals, the signals representing the left-hand front wheel speed Vw1 and the right-hand front wheel speed Vw2 are transmitted to control logic circuits 9 and 10 as signals representing first and second channel speeds Vs1 and Vs2, respectively. The lower one of the left-hand rear wheel speed Vw3 and right-hand rear wheel speed Vw4 is selected in a select-low circuit 11 and is passed to a control logic circuit 12 as a signal representing a third channel speed Vs3.
The control logic circuits 9, 10 and 12 are arranged to effect "on"-"off" control of hold valves HV and decay valves DV in the respective channels on the basis of the signals representing the respective channel speeds Vs1 to Vs3 (any of these channel speeds will be referred to simply as "channel speed Vs" hereinafter).
Signals representing the wheel speeds Vw1 to Vw4 are passed to a computed vehicle speed computing circuit 13 which comprises a select-high circuit 14 and a filter circuit or limiter circuit 15. The select-high circuit 14 is arranged to provide a signal representing the highest one of the four wheel speeds Vw1 to Vw4. The filter circuit 15 is arranged to provide, as computed vehicle speed Vv resembling the real wheel speed, a signal representing a speed having an acceleration follow-up limit restricted to +1G and a deceleration follow-up limit restricted to -1G with respect to the highest wheel speed VwH. In this case, the computed vehicle speed Vv is set up, when a state that Vv=VwH is changed to a state Vv&lt;VwH as a result of the driving wheels slipping, to increase linearly with a gradient of +1G as long as the highest wheel speed VwH remains higher than the computed vehicle speed Vv. Furthermore, the computed vehicle speed Vv is set up, when Vv becomes higher than VwH, to drops linearly with a gradient of -1G as long as Vv&gt;VwH. The computed vehicle speed Vv set up in this way is passed to the control logic circuits 9, 10 and 12.
Pressure reduction starting point judging sections 16 to 18 are provided in the respective channels to judge a pressure reduction starting point when "on" control of the decay valves is started. The pressure reduction starting judging sections 16 to 18 are connected to output terminals of the respective control logic circuits 9, 10 and 12 through notional switches SW10, SW20 and SW30, and directly to input terminals of the control logic circuits 9, 10 and 12. The functions of the pressure reduction starting point judging sections 16, 17 and 18 will be described later.
FIG. 2 shows variations in the channel speed Vs, the acceleration/deceleration dVs/dt of the channel speed Vs, and the brake hydraulic pressure Pw in each channel of the conventional anti-lock control system shown in FIG. 1, together with hold signals HS for opening/closing the hold valves HV and decay signals DS for opening/closing the decay valves DV. Similar view is also shown in U.S. Pat. No. 4,741,580.
When the brake equipment of the motor vehicle is not being operated while the motor vehicle is running, the hold valve HV remains open while the decay valve remains closed, the brake hydraulic pressure Pw is not increased; and when the brake equipment is operated, the brake hydraulic pressure Pw is rapidly increased at time t0 so that the channel speed Vs is decreased (normal mode). A reference wheel speed Vr is set up which is lower by a predetermined amount .DELTA.V than the channel speed Vs and follows the latter with such a speed difference. More specifically, reference wheel speed Vr is set up so that when the deceleration (negative acceleration) dVs/dt of the channel speed Vs reaches a predetermined threshold level, say -1.1G at time t1, anti-lock control is started, and the reference wheel speed Vr is thereafter made to linearly decrease with a deceleration gradient .theta. (=-1.1G). At time t2 when the deceleration dVs/dt of the channel speed Vs reaches a predetermined maximum value -Gmax, the hold signal HS is generated so that the hold valve HV is closed, thus holding the brake hydraulic pressure Pw.
With the brake hydraulic pressure Pw being held, the channel speed Vs is further decreased. At time t3, the channel speed Vs and the reference wheel speed Vr become equal to each other, and a first cycle of anti-lock control is started; and the decay signal DS is generated, by which the decay valve DV is opened so that reduction of the brake hydraulic pressure Pw is started. As a result of this reduction of the brake hydraulic pressure Pw, the channel speed Vs is changed from decrease to increase at time t4 when a low peak VL of the wheel speed Vw occurs. The decay signal DS is interrupted either at the time t4 or at time t5 when the wheel speed Vw is increased up to the level of a speed Vb that is higher than the low peak speed VL by 15% of the difference Y between wheel speed Va occurring at the time t3 when the reduction of the brake hydraulic pressure was started and the low peak speed VL, i.e., Vb=VL+0.15Y (FIG. 2 shows the case where the decay signal DS is interrupted at the time t4). Thus, the decay valve DV is closed so that the reduction of the brake hydraulic pressure PW is stopped and thus the brake hydraulic pressure is held. The channel speed Vs is further increased up to the level of a speed Vc that is higher than the low peak speed VL by 80% of the difference Y between the wheel speed Va occurring at the time t3 when the reduction of the brake hydraulic pressure Pw was started and the low peak speed VL, i.e., Vc=VL+0.8Y.
Subsequently, at time t7, a high peak VH of the channel speed Vs is reached; thereupon, the brake hydraulic pressure Pw is again increased. In this case, when the difference Vv-Vs between the computed vehicle speed Vv and the channel speed Vs becomes equal to a predetermined small value .DELTA.Vo before the real high peak of the channel speed Vs occurs, this is judged as if the real high peak of the channel speed Vs were reached, and thereupon buildup of the brake hydraulic pressure Pw is started. The buildup of the brake hydraulic pressure Pw is effected in such a manner that the brake hydraulic pressure Pw is alternately increased and held in succession by the fact that the hold signal HS is turned on and off mincingly, or with relatively short intervals so that the brake hydraulic pressure Pw is caused to gradually build up. In this way, the channel speed Vs is decreased, and at time t8 (corresponding to the time t3), a second cycle of the mode for reduction of the brake hydraulic pressure occurs. The time period Tx of the initial brake hydraulic pressure buildup occurring at the time t7 is determined on the basis of calculation of the average acceleration (Vc-Vb)/.DELTA.T over the time interval .DELTA.T between the time t5 and the time t6 (the average acceleration depends on the friction coefficient .mu. of the road surface), and the time period of the subsequent pressure holding or pressure buildup is determined on the basis of the acceleration or deceleration of the channel speed Vs which is detected immediately prior to the pressure holding or pressure buildup. The brake hydraulic pressure increasing, holding and reducing modes are effected in combination as mentioned above, and thus the channel speed Vs can be controlled so that the vehicle speed can be decreased, while the wheels of the motor vehicle are prevented from being locked.
Referring again to FIG. 1, notional switches Sw10 to SW30, which are provided for operating the pressure reduction starting point judging sections 16 to 18 respectively, are turned on during the period from the time point when the deceleration of the channel speeds Vs1 to Vs3 becomes higher than -1.1G (time t1 in FIG. 2) and the time point when reduction of the brake hydraulic pressure Pw is started (time t3 in FIG. 2). In the pressure reduction starting point judging sections 16 to 18, the pressure reduction starting point is determined on the basis of comparison of the channel speed Vs and reference wheel speed Vr as mentioned above; a speed V.sub.DS (V.sub.DS =Vv-.DELTA.V') which is lower than the computed vehicle speed Vv by a predetermined amount .DELTA.V' is set as pressure reduction prohibiting threshold value; and reduction of the brake hydraulic pressure Pw is prohibited when Vs&gt;V.sub.DS. Thus, in the sections 16 to 18, when the below-mentioned two conditions represented by equations (1) and (2) are both satisfied, this is judged as brake pressure reduction starting condition; thereupon the control logic circuits 9, 10 and 12 are caused to provide the decay signals DS. EQU Vs.ltoreq.Vr (1) EQU Vs.ltoreq.V.sub.DS ( 2)
The construction and operation of the conventional anti-lock control system has been illustrated and described above by way of example. It is also possible that a speed V.sub.DS ' having a predetermined slip ratio S with respect to the computed vehicle speed Vv may be set as pressure reduction prohibiting threshold, instead of the above-mentioned pressure reduction prohibiting threshold V.sub.DS. The slip ratio S is defined as S=(Vv-V.sub.DS ')/Vv; thus V.sub.DS '=(1-S)Vv.
With such a conventional anti-lock control system, however, if the highest wheel speed VwH be rapidly increased as a result of at least one of the driving wheels slipping before anti-lock control is started, then problems will arise which will be described below.
Referring to FIG. 3, description will now be made of anti-lock control operation in the first channel when the left-hand front wheel speed Vw1 is rapidly increased as a result of the left-hand front wheel, for example, one of the driving wheels being caused to slip on a road surface having a low friction coefficient while the accelerator pedal of the running motor vehicle is being depressed.
In such a case, the highest one VwH of the four wheel speeds becomes equal to the channel speed Vs in the first channel, i.e., Vs1, and the highest wheel speed VwH is caused to rapidly increase with an acceleration gradient higher than +1G. The computed vehicle speed Vv is caused to linearly increase with an acceleration gradient of +1G under the action of the filter circuit 15; thus the computed vehicle speed Vv tends to remarkably depart upwardly from the real vehicle speed. Under such a condition, if the driver's foot is put off the accelerator pedal and thereupon the brake pedal is depressed, then the channel speed Vs (=VwH) will be rapidly decreased so that dVs/dt will reach -1.1G immediately; thus anti-lock control will be started, and subsequently the condition Vs.ltoreq.Vr represented by the above equation (1) will be satisfied. Furthermore, if the highest wheel speed VwH equal to the channel speed Vs is decreased so that VwH=Vv and further decreased so that VwH&lt;Vv, then the computed vehicle speed Vv is caused to gradually and linearly decrease with a deceleration gradient of -1G. At the same time, the channel speed Vs which is rapidly decreased, is decreased toward the real vehicle speed, and, on the way, goes below V.sub.DS (=Vv-.DELTA.V') or V.sub.DS ' (=(1-S)Vv) so that the above condition represented by the equation (2) will be satisfied. In this way, reduction of the brake hydraulic pressure will be started, and the channel speed Vs will become substantially equal to the real vehicle speed. In contrast, the computed vehicle speed Vv will be gradually decreased with the deceleration gradient of -1G so that satisfaction of the brake pressure buildup starting condition, or occurrence of a high peak of the channel speed Vs is delayed (when the condition Vs.gtoreq.Vv-.DELTA.Vo is satisfied, this is judged as occurrence of a high peak of the channel speed Vs). Thus, a non-braking condition will be permitted to persist until the above brake pressure starting condition is satisfied.
Such a tendency is remarkable with a 4WD motor vehicle.